1. Field of the Invention
The present invention relates to a data communication system and a radiation image capturing system and particularly to a data communication system for carrying out transmission/receiving of data between a wireless terminal and an access point and to a radiation image capturing system utilizing such system.
2. Description of Related Art
Recently, various types of technology concerning wireless communication of data or the like carried out by use of, for example, IEEE 802.11 method between a wireless terminal and an access point (also referred to as a base unit, base station, station, or the like) in a separated position by way of a wireless local area network (wireless LAN, or also referred to as WLAN) have been developed (e.g., refer to Japanese Patent No. 4519535)
Moreover, data communication technology by way of wireless LAN has been expanding not only in the conventional wireless communication in the computer network but also in a wide range of fields such as wireless communication between wireless terminals such as a computer, audio equipment, visual equipment, and a game machine. In addition, field of utilization has been expanding, for example, into the field of medicine.
In the field of medicine, in a case where a radiation image is captured for the purpose of diagnosis or the like, a screen film using silver salt or the like has been conventionally used. However, in recent years, a radiation image capturing device for detecting radiation such as X-ray irradiated through a subject by photodiode or the like and acquiring a radiation image as a digitalized image data has been developed.
There is a so-called direct-type radiation image capturing device which generates an electric charge according to the dose of irradiated radiation by a detecting element and converts the electric charge thus generated into an electric signal (that is, image data, same shall be applied below) and a so-called indirect-type radiation image capturing device which converts irradiated radiation into a light having other wavelength such as a visible light by way of a scintillator or the like and then generates an electric charge by a photoelectric conversion element such as a photodiode according to the energy of the converted and irradiated light to convert the electric charge into an electric signal. In the present invention, a detecting element of the direct-type radiation image capturing device and a photoelectric conversion element of the indirect-type radiation image capturing device will be collectively referred to as a radiation detecting element.
Such type of a radiation image capturing device has been known as a flat panel detector (FPD) and conventionally has been configured as a dedicated machine formed as one with a supporting platform (e.g., refer to Japanese Published Unexamined Patent Application No. H9-73144). However, in recent years, a portable radiation image capturing device including a case in which the radiation detecting element and the like are stored so as to be able to be transported has been developed and practically used (for example, refer to Japanese Published Unexamined Patent Application No. 2006-058124 and Japanese Published Unexamined Patent Application No. H6-342099).
Specifically, because a portable radiation image capturing device can be carried to any location, in a case where radiation image of a patient who cannot move to a capturing chamber is captured, the radiation image capturing device can be brought into the patient's room and used by placing the device to the patient's body or sandwiching the device between a bed and the patient's body. Then, radiation is irradiated through the subject to the radiation image capturing device in such a condition to carry out capturing of a radiation image.
In this case, when image data or the like acquired by the radiation image capturing device is transmitted to an external device, if it is configured that the image data or the like is transmitted by, for example, a wired method in which a cable is connected to the radiation image capturing device, there is a possibility that the cable might entangle around an operator such as a radiology technician. Therefore, transmission of the image data or the like is carried out by a wireless method in many cases.
In such case, data communication technology by way of the above-mentioned wireless LAN is often utilized. If the wireless LAN is used, there is an advantage that configuration for transmission or the like of the image data or the like can be easily established since its standards are already determined and it is technologically matured.
However, on the other hand, technology of the wireless LAN has been widely spread as mentioned above and used by various wireless terminals. Therefore, there is a possibility that transmission/receiving of data have been already frequently carried out between various wireless terminals and access points even in the patient's room or the like.
Then, in a case where the above-mentioned portable type radiation image capturing device is brought into the patient's room in such a condition and data communication between the radiation image capturing device and an external device (to be precise, between an access point to which the external device is connected and a radiation image capturing device) is carried out, there is a possibility that radio waves from other wireless terminals become an interfering radio wave and prevent data communication between the radiation image capturing device and the external device.
Therefore, in a case where data communication is carried out by way of the wireless LAN, it becomes necessary to carry out data communication by selecting a channel which is not influenced by the interfering radio wave, or a channel which is least influenced by the interfering radio wave and by using such a channel. Moreover, concerning the influence by the interfering radio wave, in a case where wireless communication is carried out by use of a channel of the wireless LAN, it becomes necessary to give consideration to the influence of not only the interfering radio wave which uses the same channel but also the influence of so-called leakage of the interfering radio wave to the channel from a channel in the vicinity of the channel.
Therefore, in a data communication device or the like described in the Japanese Patent No. 4519535, a technology has been proposed that the number of access points which are carrying out communication by use of a channel is added and concerning a channel other than this channel, a coefficient, which decreases as the other channels become farther from the channel, is multiplied to the number of access points using the other channel, addition with weighting is carried out, and based on the values thus calculated, utilization condition of the channel is judged.
Meanwhile, 13 channels are provided to the wireless LAN of so-called 2.4 GHz band. In the band frequency of the 2.4 GHz, a radio wave used in a certain channel leaks out to another channel as mentioned above and the radio wave relatively strongly influences a channel in the vicinity of the channel using the radio wave. Therefore, it is said that in order to carry out communication in the 2.4 GHz band without interfering each other by the radio waves, only 3 or 4 channels among the 13 channels can be practically used.
In a wireless LAN of the 2.4 GHz band frequency, since the radio wave relatively strongly influences a channel in the vicinity of the channel using the radio wave as mentioned above, it becomes necessary to judge the utilization condition of a radio wave in a channel which is to be used, giving a consideration to the influence of the interfering radio wave from a channel which is relatively far from the channel to be used, similarly to a data communication device described in Japanese Patent No. 4519535.
However, it is known that in a case where wireless LAN is used in a so-called 5 GHz band frequency, influence by a radio wave reaches only within the channel used for the radio wave and channels adjacent to the channel and does not expand any further (e.g., refer to later-described FIG. 5). Therefore, it becomes hardly necessary to give consideration to the influence by the radio wave used in a channel which is farther than the channels adjacent to the channel, unlike the data communication device or the like described in the above-mentioned Japanese Patent No. 4519535.
Moreover, according to a study by the inventors of the present invention, it was understood that in a judgment of which channel to select at the time when a channel without influence by the interfering radio wave or with least influence is selected, although the level of the interfering radio wave is an important factor as described in the data communication technology described in the above-mentioned Japanese Patent No. 4519535 (that is, the number of access points utilizing the channel or the other channels), another important factor contributes to the selection.
It became known that if this important factor functions in wireless data communication, the influence is not limited to the influence by other interfering radio waves such as lowered communication speed of data or deteriorated S/N ratio of data caused by the noise which superimposes on the transmitted data, but in the worst case, there is a possibility that data communication cannot be carried out.